The proposed invention is a device whose sole purpose is to evenly divide a flow into two or more equal streams. This invention is only for free flows in pipes or containers having a liquid/gas interface under the influence of gravity, centrifugal force, or acceleration.
When a subsistence farmer in Asia wishes to divide the irrigation water into two or more parts, he simply hoes out a shallow trough in the sides of the main irrigation ditch to conduct water to lateral irrigation ditches. If two of the shallow troughs are near each other and about the same shape, length and depth (as measured from the water surface), the flow will be divided more or less evenly. This sort of liquid flow dividing is what the proposed invention is for, but the proposed invention has unique features that make it vastly superior to these hoed troughs. Both this invention and the hoed troughs are simply types of weirs, however and thus are variants on a very old concept.
The primary use for this invention is when the head at the two weirs is different. This difference might be due to vertical misalignment of the weirs, surging due to flow patterns, or other causes. In such a case division of the flow by a V-notch weir or a rectangular notch weir is generally unsatisfactory due to the large difference in the flow over the two weirs. This is especially true at low flows where there may be little or no flow over the weir with the lower head. A normal "V" notch weir will almost triple the flow if the head above the notch bottom changes by a factor of 1.5. This implies that even a slight vertical misplacement of one of the weirs would result in a considerable difference in the amount of flow through that weir. This is one of the central problems in using weirs to divide the flow.
Prior art efforts to solve these problems have included narrow notch weirs. This does minimize the problem at low flows because the total head is raised making the head differences small compared to the total head. There are at least two significant problems with this approach. First, the narrow opening is quite subject to clogging in many (if not most) open channel applications. Second, it has a very small dynamic range. That is, if it is accurate in dividing the flow at, say, 0.2 gallons per minute, the maximum flow for a device with reasonable total height might be only 0.5 gallons per minute.
In order to overcome these disadvantages prior art includes the modified combination V-notch and narrow rectangular notch weir. The lower portion of the "V" in a V-notch weir ends in a narrow rectangular notch weir which goes still lower (mainly for use at low heads). This device does solve the problem in part, but also has some disadvantages. The low flow portion (the narrow slot) is still subject to clogging. The high flow region uses the inaccurate V-notch so that a small error in vertical alignment or head will cause a significant error in flow dividing for medium flows. The present invention solves these problems in flows subject to weir clogging such as wastewater flow division, cooling water flow division, irrigation water flow division or slurry flow division.
In flows containing a clogging agent, such a buildup of the clogging agent can cause uneven distribution or malfunction of some sort. With the present invention, however, this buildup is actually used to perfect the division of flow!
This invention has particular application in septic systems. A typical system consists of a septic tank, a distribution box or "Tee", two or more subsurface absorption areas, and the associated piping. Flow division takes place in the distribution box, often made of concrete. The walls of this box have holes or knockouts through which four inch distribution pipes are inserted. The open ends of the four inch pipes serve in lieu of flow dividing weirs.
Thus if one distribution pipe is higher than another--an inevitable occurrence--the flows will be unequal. The flow rate through the lower pipe will have to reach some minimum value before the head inside the box rises enough for there to be any flow at all through higher pipes. This means that any distribution box is a very poor divider for low flow rates. Unfortunately, since the boxes are always installed so that they are downstream from a large reservoir (the septic tank), the average flow rate is very low in practice and the resulting distribution poor. Even if perfect flow is achieved when the system is first installed, later movement and settling will cause a variation in the outlet pipe elevations.
Prior art has tried to solve these problems in septic systems by dropping the effluent over a knife edge as a means of flow division as shown in U.S. Pat. No. 4,605,501 to Tyson and U.S. Pat. No. 3,497,067 to Tyson, by using dosing systems which thereby transiently increase the flow rate, and by using orifices whose height is adjustable over the pipe as shown in U.S. Pat. No. 4,298,470 to Stallings. None of these options have been successful enough to be generally accepted.